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  • Advancements on the formation mechanism of shear bands in bulk metallic glasses
    Author:   | Date:2005-11-11   | Click Rate:    | 【Close
         Recently, bulk metallic glasses (BMGs) have attracted large interest due to their unique physical, mechanical, and chemical properties. However, plastic deformation of BMGs at high stresses and low temperatures (e.g. room temperature) is prone to be localized into shear bands. This deformation behavior has limited the application of BMGs as engineering material so far. Two important hypotheses, namely free volume softening and local adiabatic heating softening, have been proposed to explain the formation mechanism of shear bands in BMGs. However, the precise physical nature of the mechanism of formation of shear bands in BMGs still remains unclear. Furthermore, it is noted that these investigations are mainly focused on quasi-static conditions. However, considerable research work has demonstrated that the formation of shear bands in BMGs is a rate-dependent process. Though great progress in understanding adiabatic shear banding instability in crystalline materials has been made during past several decades, the physical nature of adiabatic shear banding instability in BMGs has not previously been identified. So, understanding the formation mechanism of shear bands in BMGs has very important scientific and engineering values.

        Professor Dai lan-hong’s group in Institute of Mechanics, Chinese Academic of Science, studied the formation mechanism of shear bands in BMGs systematically and achieved a series of advancements:(1)Their experimental results have demonstrated that the density of shear bands increases and the serrated flow diminishes with increasing strain rate. The finding reveals that the serrated flow depends on the individual behavior of single shear bands, and the collective behavior of numerous shear bands will suppress the serrated flow.(2)Nano/micro-scale voids and cracks at the intersecting sites of shear bands and preferential etching of shear bands were observed as well under quasi-static compressive loads.

        These observations provide some evidence that the formation of shear bands in bulk metallic glasses results mainly from local free volume coalescence under quasi-static loading.(3)Based on the deformation work and adiabatic heating transformation in shear bands, the temperature increasing in shear bands was estimate to 2000K in BMGs. The temperature increasing in shear bands far exceed the melting temperature demonstrates that adiabatic heating will affect the formation of shear bands in BMGs.(4)A linear instability analysis was performed to highlight the mechanism of formation of adiabatic shear banding instabilities in bulk metallic glasses (BMGs). It is found that this instability is determined by the free volume coalescence-diffusion Deborah number. The most important findings are that both free volume coalescence softening and adiabatic heating softening exert an influence on the formation of adiabatic shear banding instability in BMGs under dynamic lading, and higher strain rate promotes the growth of instability. However, the formation of shear bands under quasi-static is fully controlled by the free volume coalescence softening and has little effect of adiabatic heating. These results are of particular significance in understanding the mechanism of formation of shear bands in BMGs.

        The above mentioned works have been published in: Applied Physics Letters,2005,87: 141916-1-3;Journal of Non-Crystalline Solids, 2005, 31:3259–3270;Material Chemistry & Physics, 2005,93: 174–177;Intermetallics, 2005, 13 : 827–832。
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